1. Field of the Invention
The present invention relates to a focus detecting device, particularly suitable for cameras with interchangeable photographing lenses.
2. Description of the Prior Art
Until now, many camera focus detecting devices have been proposed, in which the first and the second images are respectively formed by means of a pair of secondary image forming lenses behind a position conjugate the film plane in a camera. The focusing condition of the principal image forming lens is detected from the position changes of the above first and second images relative to a pair of photoelectric transducers on or near the focusing planes of the above pair of secondary image forming lenses.
However, the focus detecting devices mentioned above have inherent disadvantages. When using a principal image forming lens, particularly when an interchangeable photographing lens is mounted on a single lens reflex camera, correct focus detection is difficult because of vignetting due to the large iris of the photographing lens.
The shortcomings of the conventional devices will be explained with reference to FIG. 1. FIG. 1 is a schematic view of the distance measuring optical system of such focus detecting devices. In the drawing, the light beam having passed the photographing lens 1 is converged upon the expected focus plane 2 and is then diverged. A part of the light beam having passed the photographing lens 1 is converged and imaged upon the light receiving planes of a pair of photoelectric transducers 4, 4' consisting, of photoelectric transducing elements, by means of a pair of secondary image forming lenses 3, 3' arranged symmetrically with respect to the optical axis of the photographing lens 1. The pair of photoelectric transducers 4, 4' serve to detect the position of the image and consist of a series of photoelectric transducing elements. It is important that the brightness distribution on the light sensing surfaces of the photoelectric transducers 4, 4' be equal for a certain object when such focus detecting devices are used. The reason is that focus detection is carried out by comparison of photoelectric outputs of the photoelectric transducers 4, 4' for the same portion of the image.
In FIG. 1, the light beams incident on the upper end light receiving surface 4a, the center light receiving surface 4c and the lower end light receiving surface 4b of the first photoelectric transducer 4 are projected reversely from these light receiving surfaces of the iris of the photographing lens 1 through the secondary image forming lens 3. Thus, the light beam having the divergence of a1, a2 and a3 falls on the upper end light receiving surface 4a and the light beam having the divergence of c1, c2 and c3 falls on the center light receiving surface 4c. The divergence of a1, a2 and a3 and that of c1, c2 and c3 are determined by the effective F-number of the secondary image forming lens 3 and are almost equal to each other. However, when the light beam is reversely projected on the lower end light receiving surface 4b just as on the former two light receiving surfaces, the light beam falling on the lower end light receiving surface 4b has the divergence of b1, b2 and b3. And when the iris of the photographing lens 1 is small, part of the light beam is vignetted at the upper end of the photographing lens 1 so that only the light beam of the divergence b1, b2 and b3 falls on the lower end light receiving surface 4b. This means that when an object having an even brightness distribution is measured, the brightness on the upper end and central light receiving surfaces 4a, 4c is lower than that on the lower end light receiving surface 4b. Thus, it is impossible to obtain the brightness distribution corresponding to that of the object on the light receiving surface of the photoelectric transducer.
This phenomenon also takes place in connection with the second photoelectric transducer 4' and the brightness on the upper end light receiving surface 4'a on which the light beam which is vignetted at the lower end portion of the photographing lens 1 falls is lower than that on the center surface 4'b and lower end surface 4'c. Here, the image formed on the lower end light receiving surface 4b is the same as that formed on the upper end surface 4'a, while the brightness reproductivity of the object image is different depending upon the portions of the object, so that the brightness distribution of the images upon both photoelectric transducers 4, 4' is not same. Consequently, it is impossible to correctly compare the outputs of the photoelectric transducing elements of the photoelectric transducers, 4, 4', thus resulting in errors in focus detection.
In order to overcome this disadvantage, a device has been proposed, as disclosed in U.S. Pat. No. 4,322,616, which restricts the aperture of the secondary image forming lenses 3, 3' with the shading plates 5, 5' according at the smallest F-number of the photographing lens 1 compensating for the focus detection error due to vignetting. However, in this prior art device, the pin for transferring the smallest F-number of the photographing lens to the camera directly contacts the cam plane on one of the shading plates 5, 5' so as to vary the aperture of the secondary image forming lenses 3, 3', depending on the pin position for transferring the smallest F-number, so that the following disadvantages have been confronted.
In such a focus detecting device, it is necessary that the apertures of the pair of the secondary image forming lenses 3, 3' always be symmetrical so that the mechanism is accurate in keeping the cam plane and the shading plate 5, 5' in linking engagement in such a manner that the pair of shading plates 5, 5' are always symmetrical. This requires very troublesome assembly and adjustment.
Also in the prior art device, the position of the shading plates changes every time the smallest F-number of the photographing lens changes so that adjustment is all the more troublesome. In fact, it is ideal to compensate for the aperture of the secondary image forming lenses 3, 3' every time the smallest F-number of the photographing lens 1 changes. However, when the smallest set F-number is in a certain determined range, even if the positions of the shading plates 5, 5' are fixed, the vignetting influence is negligible. Building such a device into the camera, is inconvenient because of the troublesome assembly required.
It is accordingly an object of the present invention to provide a highly practicable focus detecting device.